At present, as a secondary battery, a nonaqueous secondary battery in which a separator made of resin is impregnated with a nonaqueous electrolytic solution containing an organic solvent is mainly used. Here, the nonaqueous electrolytic solution containing the organic solvent is not completely immobilized in the separator. Thus, when the battery is broken, the nonaqueous electrolytic solution could leak.
Therefore, instead of the nonaqueous secondary batteries, solid-type secondary batteries using solid electrolytes such as ceramics and polymers are actively studied and developed.
For example, as described in Patent Literature 1, a solid-type secondary battery using a solid electrolyte mainly composed of a sulfide is considered. Here, the sulfide exhibits a characteristic that the conductivity of lithium ions serving as a charge carrier is relatively high. In addition, such a sulfide material is advantageous in that the sulfide material is excellent in moldability because the sulfide material is relatively soft, and an interface between the sulfide material and an active material used for an electrode is easy to be formed. For example, simply by pressurizing a mixture of the sulfide material and the active material, the sulfide material and the active material are brought into close contact with each other, to form the above-mentioned interface. Thus, lithium ion conductive paths are easily ensured.
However, when the solid electrolyte mainly composed of a sulfide is used as the separator, particles are brought into contact with one another by being pressurized, and thus, sparse portions are generated. Then, when the solid-type secondary battery using the solid electrolyte as the separator is activated, lithium ions are concentrated in the above-mentioned sparse portions. As a result, a problem of lithium metal dendrites being formed is caused. In order to solve this problem, the thickness of the separator composed of the solid electrolyte has to be increased. In addition, sulfides are known to generate hydrogen sulfide, having a bad smell, by reacting with water. Thus, when the solid electrolyte is used, how to suppress generation of hydrogen sulfide is a problem.
In order to solve the above problems, use of an oxide as the solid electrolyte is also considered. Usually, an oxide is subjected to sintering at a high temperature, to be used as a solid electrolyte. Such a solid electrolyte composed of an oxide has high density, thus, is a dense structural body, and thus, is less likely to cause a problem of dendrite. In addition, oxides are relatively stable chemically, and do not generate hydrogen sulfide.
As described in Non-Patent Literature 1, in recent years, Murugan, Weppner, et al. proposed Li7La3Zr2O12 which is an oxide including a garnet crystal structure. Non-Patent Literature 1 indicates that: LiOH, La2O3, and ZrO2 were mixed together; and then, a reaction product obtained by subjecting the mixture to heat treatment was annealed at 1230° C. for 36 hours, whereby Li7La3Zr2O12 was produced. In addition, Non-Patent Literature 1 indicates that, in order to prevent loss of lithium, the rate of temperature increase was set to 1° C./min. Furthermore, Non-Patent Literature 1 indicates that when impedance of Li7La3Zr2O12 was measured, particle boundary resistance and material resistance were observed, and further indicates that Li7La3Zr2O12 was capable of conducting lithium ions.
Patent Literature 2 indicates that Li7La3Zr2O12 described in Non-Patent Literature 1 is in a particulate form, and further provides specific description of a solid electrolyte using Li7La3Zr2O12 having an average particle diameter of 3 μm.
Patent Literature 3 indicates that: La2Zr2O7, Li2CO3, and La(OH)3 were mixed together; and the mixture was heated at 750° C. or 800° C., whereby Li7La3Zr2O12 including a garnet crystal structure was produced.